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Discuss Cable Calculations?! in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. Emily B
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    Emily B New EF Member

    Location:
    Wales
    Looking at putting a lathe into a workshop.. 16A, 3 phase, a run of 19metres at approx 35 degrees.

    Also, a 3 phase, arc welder which I was going to put into a 32A socket outlet. 7 metre run, again at 35 degrees.

    Can someone help with how I would complete a cable calculation for either?! Completely confused with three phase systems!

    Cable will be run on perforated tray in an empty (non insulated) ceiling void.
     
  2. Stufin135
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    Stufin135 Active EF Member

    Location:
    Hornchurch
  3. Emily B
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    Emily B New EF Member

    Location:
    Wales
    I need the written working's out to justify my selection! :( thanks though!
     
  4. cliffed
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    cliffed Regular EF Member

    Location:
    Lancs
    Get away with 2.5mm for 16 amp,
    6.00mm for 32 amp,conventional circuits ,in my calculations.
    Swa cable I would suggest.
     
  5. cliffed
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    cliffed Regular EF Member

    Location:
    Lancs
    On hols at moment,no homework for me.
     
  6. Spoon
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    Spoon Trusted Advisor

    Location:
    Lancashire
    Hi Emily.
    Is this a homework question?
    Tell us what you think. Show us your calcs and we are always happy to help.
    We don't like just telling people the answer, as they don't learn anything.
     
    • Agree Agree x 4
  7. Emily B
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    Emily B New EF Member

    Location:
    Wales
    Not a homework question, been given it by my boss and told to 'figure it out'.. I can complete single phase.. but unsure of how to calculate the design current at the start for 3 phase? I know the rest thing with correction factors etc
     
  8. Spoon
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    Spoon Trusted Advisor

    Location:
    Lancashire
    You don't need to calculate the current.... do you?
    You have a 16A supply and a 32A supply. I presume the breaker for the arc welder will be 32A? You just say you are using a 32A socket outlet.
    So all you need to do is select the correct size cables.
    Or am I missing something?
     
    • Bad Spelling Bad Spelling x 1
  9. MODECSOFT
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    MODECSOFT Electrical Design Software www.electricalom.com Sponsor

  10. Spunkywads26
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    Spunkywads26 Electrician's Arms

    Location:
    Birmingham
    Its been a while but here goes:

    Ib = Power (W) / Root 3 x Uo (V) x p.f

    Where:

    Ib = Design Current
    Uo = Nominal Voltage (400V)
    p.f. = Power Factor (Usually motors)

    Then you select Device rating, In

    Then you calculate Iz by applying correction factors : Iz = In/Cf

    Ib < In < Iz

    This is then used to size cable for current carrying capacity. Using this cable information you calculate the expected voltage drop:

    VD(V) = mV/A/m x Ib x l
    1000

    This is where you would use your design current, but if you used In instead it would give the maximum expected volt drop for that circuit with the selected cable type.

    Please be kind if i'm wrong with what I have posted as I believe that everyday is a school day.
     
  11. gazdkw82
    Online

    gazdkw82 Trainee Trainee Access

    Location:
    leicester
    Don't forget to include any correction/derating factors for your chosen cable
     
  12. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    party :ppooper.
     
    • Like Like x 1
  13. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    It equal to or greater than
    In/ ca cc cg ci

    Without overload
    Ib/ ca cc cv ci

    Iz=It ca cc cg ci

    And basically any other correction factor such as underground soil resistivity depth and such
     
  14. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    what's soil resistivity got to do with it. you don't send current through soil.(hopefully)
     
  15. Ian1981
    Offline

    Ian1981 Electrician's Arms

    Location:
    North east
    Just mentioning different correction factors involved in cable calcs and if buried in soil it's one of them.
    Obviously some aren't always required :)
     
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